J. Phys. Chem. C 2010, 114, 15207–15211
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Location of Ti Catalyst in the Reversible AlH3 Adduct of Triethylenediamine Dennis D. Graham,† Jason Graetz,‡ James Reilly,‡ James E. Wegrzyn,‡ and Ian M. Robertson*,† Department of Materials Science and Engineering, UniVersity of Illinois, 1304 West Green Street, Urbana, Illinois 61801, and BrookhaVen National Laboratory, P.O. Box 5000, Upton, New York 11973-5000 ReceiVed: May 16, 2010; ReVised Manuscript ReceiVed: July 27, 2010
AlH3, a metastable binary hydride with a hydrogen content of 10.1 wt % hydrogen and a density of 1.48 g/mL, is a potential lightweight hydrogen storage system for transportation applications. A key component to understanding the discharge and uptake of hydrogen is the role of the Ti dopant in these processes. Here, the morphological and compositional changes caused by the synthesis and dehydriding of the AlH3 adduct of triethylenediamine was determined by using a combination of scanning electron and scanning transmission electron microscopy as well as X-ray energy dispersive spectroscopy. It is shown that there is significant loss of the added Ti at each step in the synthesis; the Ti agglomerates in the Al particles, causing enrichment, and the amount of Ti needed to catalyze the activity is on the order of 0.4 at.%, which amounts to approximately a 90% reduction from the amount added. 1. Introduction A key challenge facing the move to a hydrogen-based economy, as applied to the transportation sector, is the discovery of a lightweight solid-state storage medium capable of regeneration and that possesses the thermodynamic and kinetic properties needed to ensure compatibility with fuel cells.1 A candidate material is alane, AlH3, which is a covalently bonded metastable binary hydride that can contain 10.1 wt % hydrogen and has a density of 1.48 g/mL. AlH3 can be synthesized in either an ethersolvated form2 or a nonsolvated form.3,4 Numerous phases (R, R′, β, γ, δ, ε, and ζ) have been identified;3 all are thermodynamically unstable under ambient conditions and are metastable at room temperature. All phases of AlH3 are reactive and have a hydrogen content approaching 10 wt %. The R-, β-, and γ-phases decompose readily at ∼100 °C at a rate and pressure suitable for transportation applications. Of these phases, the low pressure R-phase is of primary interest for automotive applications. The decomposition of crystalline R-AlH3 occurs in one step:5
3 AlH3 f Al + H2 2
(1)
Although the dehydriding reaction occurs readily, the reverse reaction does not. The Helmholtz and Gibbs free energy of formation of R-AlH3 at 298K are -9.9 and 48.5 kJ/mol AlH3, respectively.6,7 The latter value yields a room temperature equilibrium hydrogen fugacity of 50.7 GPa, which is equivalent to a hydrogen pressure of approximately 709 MPa.8 On the basis of these thermodynamic values, the direct regeneration of AlH3 from spent Al with gaseous H2 is impractical. Ashby9 directly synthesized the AlH3 adduct of triethylenediamine (TEDA) using activated aluminum powder and TEDA in tetrahydrofuran (THF) at a hydrogen pressure of 34 MPa. * To whom correspondence should be addressed. Tel: 217-333-6776. Fax: 217-333-2736. E-mail:
[email protected]. † University of Illinois. ‡ Brookhaven National Laboratory.
Although this was successful in the production of AlH3, the reversibility of this system was limited. Recently, Graetz et al. demonstrated that Ti-doped Al powder would react reversibly at relatively low pressure with TEDA to form an AlH3 adduct.10 The reaction may be written as follows:
Al(Ti) + TEDA + 3/2H2 T Al(Ti)H3-TEDA
(2)
In this paper, the results of a compositional and structural analysis of the reversible AlH3-TEDA system as well as the precursor materials are presented. The primary emphasis is on identification of the location of the Ti catalyst. 2. Experimental Procedures The TEDA-AlH3 studied was produced following the procedure developed by Graetz et al.10 First, the synthesis route of Brower et al.,3 to prepare AlH3 in an ether solvent, was modified by the addition of ethereal TiCl3 to give a final Al0.98Ti∼0.02 ratio. Upon removal of the solvent by drying for 1 h at 80 °C, a black, activated Al* powder was produced. It should be noted Al* is highly pyrophoric in air and must be handled with caution. An alternate method involved the addition of liquid Ti(OCH2CH2CH2CH2)4 [written as Ti(OBu)4 henceforth] directly to the THF solvent in which the adduct synthesis was carried out. In this case, the Al component was prepared as above but without the addition of TiCl3. The Al powder was loaded into a hydrogen charging apparatus with TEDA and THF [with Ti(OBu)4 in the alternate route] in solution, and the hydrogen uptake was measured. For this study, several samples of the alane-amine were generated using this synthetic technique. This included two Al aggregates synthesized using Ti introduced via the above methods. In one case, AlH3-TEDA product was prepared using Al(0.02Ti) and then discharged to reprecipitate the Al. The materials studied are summarized in Table 1. The morphology and microstructure as well as the elemental distribution in these samples were determined by using either a scanning electron microscope (SEM) or a scanning transmission
10.1021/jp104453w 2010 American Chemical Society Published on Web 08/17/2010
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J. Phys. Chem. C, Vol. 114, No. 35, 2010
Graham et al.
TABLE 1: Summary of Materials Analyzed material
dopant
activated Al prepared using TiCl3 activated Al prepared usingTi(OBu)4 AlH3-TEDA Al residue from decomposition of AlH3-TEDA at 80 °C
2 mol % Ti 2 mol % Ti 0.1 mol % Ti 2 mol % Ti
electron microscope (STEM). The SEM used was a JEOL 7000F equipped with a Schottky field-emission electron source and with a ThermoElectron X-ray energy dispersive spectrometer. The STEM used was a JEOL 2010F, which is also equipped with a Schottky field-emission source and an Oxford EDS detector with an atmospheric thin window. The electron microscope was operated at an accelerating voltage of 200 keV. To prepare the samples for study, they were pressed into either carbon tape on a SEM stub or a holey carbon TEM film; this procedure was conducted inside an Ar-filled glovebox maintained at 0.1 ppm nominal H2O and O2. No effort was made to prevent oxygen exposure before loading the stub into the SEM except to minimize the transfer time from the glovebox to the microscope; typically, this time was less than 5 min. In the case of the TEM samples, a Gatan HHST 4004 environmental transfer stage was used to minimize exposure of the sample to the environment during transfer from the glovebox to the microscope. The sample was loaded in the stage, and the stage tip was retracted into the transfer chamber, which was evacuated to a roughing pump vacuum before the stage was transferred from the glovebox and inserted in the microscope column; this method has proven effective at minimizing degradation of air sensitive samples.
Figure 1. Morphology of particles produced using the different dopants. (a) Secondary electron image of TiCl3-doped activated Al(Ti) precursor, (b) higher magnification of the boxed area, (c) secondary electron image of 2 mol % Ti(OBu)4-doped activated Al(Ti) precursor, and (d) higher magnification image of the boxed region.
3. Results The morphology and composition of the activated precursor material with Ti introduced through either 2 mol % TiCl3 or 2 mol % Ti(OBu)4 are compared and contrasted in the scanning and transmission electron micrographs and composition maps shown in Figures 1-4. The SEM images presented in Figure 1 for the precursor materials produced with either dopant show that the particles either coalesce to form an aggregate or exist as large flat particles. The only discernible difference between the materials synthesized with the different dopants was in the size of the particles; it was approximately an order of magnitude larger in the precursor produced with Ti-O(Bu)4 than with TiCl3. Compositional analysis of a flat and of a nodular particle, Figure 2, shows that both are Al with the nodules containing a trace of Ti. Similar compositional variations were found for precursors prepared with TiCl3. No evidence for significant segregation of any of the elements was detected in the images formed with backscattered electrons, which suggests that within the detection limit, the composition is homogeneous, and no clusters of Ti have formed. To determine if the composition was inhomogeneous on a finer scale, the precursor materials prepared with both dopants were examined in a STEM. The results are summarized in Figures 3 and 4 for the 2 mol % TiCl3 and 2 mol % Ti(OBu)4doped material, respectively. The bright-field STEM micrograph of a TiCl3-doped AlH3 precursor aggregate along with elemental energy dispersive spectroscopy maps for Al, Cl, and Ti are presented in Figure 3. From this figure, it can be seen that the aggregate was comprised of many particles with an average size of 26 nm. From a comparison of the elemental maps, it is evident that the Ti and Cl distributions follow that of Al. Although the maps suggest the existence of Ti-rich clusters, this is a
Figure 2. EDS spectra from the flat particle and from nodules seen in Figure 1d. Material: 2 mol % Ti(OBu)4-doped activated Al(Ti) precursor. The locations of Ti and Cl peaks are indicated.
consequence of the variable thickness of the aggregate as evidenced by the contrast variations seen in the image and confirmed through determination of the concentration, which within experimental error was uniformly ∼1.2 at. %. Similarly, as shown in the images and maps presented in Figure 4, for precursor material doped with Ti-O(Bu)4, Ti was uniformly distributed at a level of ∼0.5 at. %. The difference in particle size observed in the SEM images is confirmed in the STEM imagessThe average size was 260 nm for the Ti-O(Bu)4-doped system as compared to 26 nm for the TiCl3-doped one. The morphology of the reversible, hydrided AlH3-TEDA adduct shows a very distinctive morphology difference from its precursors. The aggregate structure is replaced by ribbons that have a length of several tens of micrometers and were thin (
